While I do think the original Xbox is underrated I'll have to give it to 360 for the huge library of games and superior controller.
I'd rank them:
1) X360 - a lot of great exclusive content, superior third party games, great controller
2) Xbox - clumsy controller aside, Xbox supported lots of great PC ports and two great Halo games
3) Xbox One - third party games are now largely inferior and Microsoft studios are closed or underperforming